The present invention relates to the control of echo on telephone circuits and to voice-activated switches which have been used to block noise and activate channels in telephone communications systems. (As used herein a "circuit" includes all the paths for transmission and reception of signals between talkers or telephone users in a conversation, while a "channel" is the transmission path for signals in a single direction. A circuit therefore can be comprised of at least two channels, and a channel can be comprised of all media, such as cables and satellite links, often called "trunk lines", which provide one-way transmission for signals.) The invention is particularly relevant to those systems which employ speech interpolation techniques for combining speech signals from several circuits in a single channel.
Noise, which typically is present on all communications channels, but at a significantly lower level than speech, must be minimized to provide efficient and coherent voice communication. Typically, voice-activated switches are used to recognize speech in the presence of noise in telephone communication circuits; they allow those signals that exceed a predetermined threshold level to pass into the circuit. The threshold level of the voice-activated switch is selected to be responsive to signals above the level of noise expected to be encountered on a telephone circuit. In some types of voice-activated switches the threshold level is fixed at a predetermined value, while other types, called adaptive voice switches, employ associated control circuits which enable the threshold to be continuously adjusted to remain at a level just above the noise. The inability of voice switches to detect the presence of all speech results in a "clipping" of the conversation when speech signals fall below the the predetermined noise threshold. The more complex adaptive threshold switches tend to "clip" the transmitted signals less than those having fixed thresholds since the adaptive threshold is always no higher than is necessary above the instantaneous noise level. Examples of these several types of voice-activated switches may be found in U.S. Pat. Nos. 4,008,375 (Lanier) entitled "Digital Voice Switch for Single or Multiple Channel Application"; 3,890,467 (Scuilli) entitled "Digital Voice Switch for Use with Delta Modulation" and 3,832,491 (Scuilli et al) entitled "Digital Voice Switch with an Adaptive Digitally-Controlled Threshold". Also see U.S. Pat. No. 4,052,568 (Jankowski) entitled "Digital Voice Switch".
In addition to apparatus for reducing noise, systems for controlling echo in telephone circuits have been developed because perceived echo is offensive to telephone users and, in certain types of communications systems, all echo signals (including those that may not be noticeable to telephone users) will tend to degrade communications system performance in other ways. This is particularly true of speech interpolation communications systems which will be discussed below.
Echo is generated at the two-to-four wire-interface, called a hybrid, which couples individual telephones into a telephone circuit. Because of impedance mismatches, typically created by variation in lengths of the wires connecting individual telephones to the hybrids, partial reflections of signals arriving at the hybrid will occur. The reflection of received signals back to a transmitting telephone results in echo on the circuit connecting two telephones. If the physical length of the channels which interconnect the hybrids in a telephone circuit are sufficiently long, the propagation time for signals in the circuit will be large enough for the echo to be noticed by the telephone user. If the physical channel (trunk line) length is short, some echo may still be present, although it may go unnoticed by the telephone user.
In telephone communication systems, echo signals which may be present tend to resemble low level speech signals. The echo signals are frequently at an amplitude level greater than the voice switch threshold level set to reject noise. Thus, to a conventional voice switch, many echo signals would be indistinguishable from low level speech signals. Because conventional voice switches must be designed to be sensitive to a varied range of speech levels in order to accommodate high and low level speakers, they generally will respond to an echo signal as if it were speech.
In order to deal with the problems associated with echo, telephone communications systems have employed echo suppressors in each telephone circuit. Echo cancellers which provide superior performance to echo suppressors, also can be used. However, use of either cancellers or suppressors adds significant cost to the construction of a telephone system.
Echo suppressors in general block echo signals by inserting an attenuation in the transmit channel for a telephone in a telephone circuit to reduce the echo. They are commonly used in both the transmit and receive channels of a long distance communications circuit. These devices are successful in reducing echo. However, because echo suppressors attenuate transmit speech signals when signals are present on the receive channel, when both speakers on a telephone circuit are talking simultaneously (doubletalk), there are noticeable changes in level of the speech signals which can prove disturbing to the talkers. For example, during doubletalk, when one speaker starts or stops talking, a distinct and frequently annoying change in the speech level of the other speaker can be heard. Because of these frequent changes in channel attenuation during doubletalk situations, use of suppressors can result in confusing and offensive speech transmission. Echo cancellers, on the other hand, do not present this difficulty since they remove echo from the transmit channel by generating a signal from the receive speech, which is a replica of the echo. The canceller then subtracts this echo replica signal from the transmit signal so as to cancel the echo. Echo cancellers have fewer of the limitations of the suppressors, but are substantially more complex and expensive.
In addition to being useful in controlling noise, voice switches also are useful in communications systems employing forms of speech interpolation, wherein each trunk line may carry speech signals from parties to more than one conversation. Interpolation of speech signals from several conversations is permitted by the usual pauses or breaks in speech activity by one or both parties to each conversation. One technique of interpolation currently used in telephone communications is called digital speech interpolation (DSI). An article entitled "Digital Speech Interpolation", by S. J. Campanella published in the Comsat Technical Review, Vol. 6, No. 1, and U.S. Pat. Nos. 3,542,956 (Sekimoto) entitled "PCM Telephone Communication System"; 3,927,268 (Sciulli et al) entitled "Speech Predictive Encoded Communication System"; and 3,988,674 (Sciulli) entitled "Frame Synchronization for Speech Predictive Encoded Communications Systems" furnish descriptions of several types of DSI implementations.
In interpolation systems, voice switches serve to sense the presence or absence of speech signals in several conversations and thus control the transmission of signals on a single shared circuit. In many interpolation systems, including DSI, a voice-activated switch is used not only to detect the presence of speech for transmission but also to perform its usual function of blocking noise, especially that typically found on the idle channels. However, since echo signals are generally of a higher magnitude than the noise level in a circuit, they may exceed the noise-proof threshold of the switch. Since echo signals would be passed by a conventional voice-activated switch used in interpolation systems, the interpolation system will experience extra loading due to the transmission of the echo signal. During periods of heavy traffic, the overall system performance will be degraded due to the artificial activation of channels by voice switches responding to echo signals. Even those echo signals which are not noticeable to telephone users can also act to degrade interpolation system performance. These inaudible echo signals can be encountered in long distance circuits (such as those provided by cable or satellites) and can be present in relatively short transmission paths as well.
There is therefore a need to provide a means whereby the voice switches used in interpolation systems are able to block echo signals while at the same time being sufficiently sensitive to low level talkers. Further, all telephone communications systems, not just interpolation systems, require a new means of echo control which operates during doubletalk conditions in a less offensive manner than echo suppressors, but which also is less expensive and less complex than an echo canceller.
The present invention seeks to meet these needs by means of a circuit for appropriately controlling a voice switch during those circuit conditions which allow echo to occur. Although particularly useful in interpolation system applications such as DSI, it provides a simple, effective means of echo control in a great number of communications circuit applications, providing an alternative to echo cancellers and echo suppressors.